Steer-by-wire steering apparatus with actuatable mechanism

ABSTRACT

A steering apparatus ( 10 ) for turning steerable wheels of a vehicle in response to rotation of a vehicle steering wheel ( 12 ) comprises a first assembly ( 16 ), a second assembly ( 26 ), and a mechanism ( 108 ). The first assembly ( 16 ) is operatively coupled to the steering wheel ( 12 ) and includes components ( 18  and  22 ) for monitoring applied torque and angular rotation of the steering wheel. The second assembly ( 26 ) includes a steering gear ( 34 ) and components ( 28  and  30 ) for receiving the first signal and actuating the steering gear ( 34 ) in response to the first signal. The mechanism ( 108 ) when in a first mode of operation provides a mechanical connection between the steering wheel ( 12 ) and the steering gear ( 34 ). When the mechanism ( 108 ) is in the second mode of operation, the steering wheel ( 12 ) and the steering gear ( 34 ) lack a mechanical connection.

TECHNICAL FIELD

The present invention relates to a steering apparatus for turning thesteerable wheels of a vehicle in response to rotation of a vehiclesteering wheel.

BACKGROUND OF THE INVENTION

Power steering gears are common in modern vehicles. Typically, one ormore rigid shafts connect a vehicle steering wheel to an input shaft ofthe power steering gear. The rigid shafts must be routed from thevehicle steering wheel to the input shaft of the power steering gear.Routing the rigid shafts between the steering wheel and the steeringgear is often difficult, as other vehicle components must not interferewith the shafts.

Some known vehicle steering systems have eliminated the rigid shafts.Such systems are commonly referred to as “steer-by-wire” systems. Insteer-by-wire systems, there is no mechanical connection between thesteering wheel and the steering gear. Instead, an assembly associatedwith the steering wheel sends an electronic signal to an assemblyassociated with the steering gear. The electronic signal actuates thesteering gear. Since steer-by-wire systems have no mechanicalconnection, routing of the rigid shafts between the steering wheel andthe steering gear is avoided. However, with no mechanical connection,steering control of the vehicle is lost if the steer-by-wire systemfails.

SUMMARY OF THE INVENTION

The present invention is a steering apparatus for turning steerablewheels of a vehicle in response to rotation of a vehicle steering wheel.The apparatus comprises a first assembly, a second assembly, and amechanism. The first assembly is operatively coupled to the steeringwheel. The first assembly includes components for monitoring appliedtorque and angular rotation of the steering wheel and for transmitting afirst signal indicative of the applied torque and angular rotation ofthe steering wheel. The second assembly includes a steering gear for,when actuated, turning the steerable wheels of the vehicle andcomponents for receiving the first signal and actuating the steeringgear in response to the first signal. The mechanism has first and secondmodes of operation. The mechanism, when in the first mode of operation,provides a mechanical connection between the steering wheel and thesteering gear for enabling manual actuation of the steering gear. Whenthe mechanism is in the second mode of operation, the steering wheel andthe steering gear lack a mechanical connection for enabling manualactuation of the steering gear.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will becomeapparent to those skilled in the art to which the present inventionrelates upon reading the following description with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic illustration of a vehicle steering apparatusconstructed in accordance with the present invention in a conditionproviding a mechanical connection between a steering wheel and asteering gear;

FIG. 2 is a schematic illustration of a vehicle steering apparatusconstructed in accordance with the present invention in a conditionwithout a mechanical connection between the steering wheel and thesteering gear;

FIG. 3 is a schematic elevation view, partially in section, through thesteering gear of the vehicle steering apparatus of FIG. 1;

FIG. 4 is a cross-sectional view taken approximately along line 4—4 ofFIG. 3;

FIG. 5 is a cross-sectional view taken approximately along line 5—5 ofFIG. 1;

FIG. 6 is a schematic illustration of a portion of a clutch of thevehicle steering apparatus of FIG. 1 shown in a first condition ofengagement; and

FIG. 7 is a schematic illustration of a portion of a clutch of thevehicle steering apparatus of FIG. 1 shown in a second condition ofengagement.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically illustrates a vehicle steering apparatus 10constructed in accordance with the present invention. The vehiclesteering apparatus 10 includes a vehicle steering wheel 12. The steeringwheel 12 is of known construction and is manually rotatable by a vehicleoperator.

A shaft 14 is fixed to the center or hub of the steering wheel 12.Angular rotation of the steering wheel 12 results in an equivalentangular rotation of the shaft 14. The shaft 14 extends from the steeringwheel 12 through a first assembly 16.

The first assembly 16 is operatively coupled to the vehicle steeringwheel 12 via the shaft 14. The first assembly 16 includes atorque/position sensor 18, a first electric motor 20, and a firstelectronic control unit 22. The first assembly 16 is integrated into asingle unit through which the shaft 14 passes.

The torque/position sensor 18 of the first assembly 16, shownschematically in FIG. 1, is operable to sense operator applied torqueand angular rotation of the steering wheel 12. The torque/positionsensor 18 also generates signals indicative of the applied torque andangular rotation of the steering wheel 12. The torque/position sensor 18may be any known sensor or group of sensors for sensing applied torqueand angular rotation of the steering wheel 12 and for generating signalsindicative of the sensed parameters. In one embodiment, thetorque/position sensor 18 is an optical sensor of known construction.

The first electric motor 20 is connected to the shaft 14. Preferably, agear assembly 24 connects an output of the first electric motor 20 tothe shaft 14. The first electric motor 20 is actuatable to provideresistance to rotation of the steering wheel 12 and thus, is commonlyreferred to as a “steering feel motor.”

The first electronic control unit 22 is operatively coupled to thetorque/position sensor 18 and to the first electric motor 20. The firstelectronic control unit 22 receives the signals indicative of theapplied torque and angular rotation of the steering wheel 12 from thetorque/position sensor 18. In response to the signals from thetorque/position sensor 18, the first electronic control unit 22generates and transmits a first signal corresponding to the sensedtorque and angular rotation of the steering wheel 12 sensed by thetorque/position sensor 18.

The second assembly 26 includes a second electronic control unit 28, asecond electric motor 30, a torque/position sensor 32, and a hydraulicpower steering gear 34 for turning the steerable wheels (not shown) ofthe vehicle (not shown). Alternatively, an electric power steering gearmay be used. If an electric power steering gear is used, the secondelectric motor 30 is eliminated and the electric motor of the electricpower steering gear is controlled by the second electronic control unit28 to turn the steerable wheels of the vehicle. The components of thesecond assembly 26 are integrated into a single unit.

The second electronic control unit 28 receives the first signal from thefirst electronic control unit 22. The second electronic control unit 28is further operatively coupled to the second electric motor 30. Thesecond electronic control unit 28 controls the operation of the secondelectric motor 30 in response to the first signal.

The second electric motor 30 has an output shaft that is connected withan input shaft 40 of the power steering gear 34. A gear assembly 36 maybe used to connect the output shaft of the second electric motor 30 tothe input shaft 40 of the power steering gear 34. The second electricmotor 30, upon receiving a signal from the second electronic controlunit 28, is operable to actuate the power steering gear 34.

The power steering gear 34 is an integral hydraulic power steering gear34. Other steering gears are contemplated by this invention, such asrack and pinion steering gears and electric power steering gears. Theintegral hydraulic powered steering gear 34 is illustrated in FIG. 3.

The power steering gear 34 includes a housing 42 and a drive mechanism44. The drive mechanism 44 is moved in response to rotation of the inputshaft 40 of the power steering gear 34. Motion of the drive mechanism 44results in a turning of the steerable wheels of the vehicle.

The drive mechanism 44 includes a sector gear 46 having a plurality ofteeth 48. The sector gear 46 is fixed on an output shaft 50 that extendsoutwardly through an opening in the housing 42 of the power steeringgear 34. The output shaft 50 is typically connected to a pitman arm (notshown) that is connected to the steering linkage (not shown) of thevehicle. Thus, as the sector gear 46 rotates, the output shaft 50 isrotated to operate the steering linkage. As a result, the steerablewheels of the vehicle are turned.

The power steering gear 34 further includes a hydraulic motor 52 formoving the drive mechanism 44. The hydraulic motor 52 is located withinthe housing 42 of the power steering gear 34. The housing 42 of thepower steering gear 34 has an inner cylindrical surface 54 defining achamber 56. A piston 58 is located within the chamber 56 and divides thechamber 56 into opposite chamber portions 60 and 62. One chamber portion60 is located on a first side of the piston 58 and the other chamberportion 62 is located on a second side of the piston 58. The piston 58creates a seal between the respective chamber portions 60 and 62 and iscapable of axial movement within the chamber 56.

A series of rack teeth 64 is formed on the periphery of the piston 58.The rack teeth 64 act as an output for the hydraulic motor 52 and meshwith the teeth 48 formed on the sector gear 46 of the drive mechanism44. When the piston 58 moves axially, the rack teeth 64 of the piston 58interact with the teeth 48 of the sector gear 46 to rotate the sectorgear 46.

A pump (not shown) supplies hydraulic fluid from a reservoir (not shown)to the hydraulic motor 52. Typically, the engine (not shown) of thevehicle drives the pump. However, the pump could be driven otherwise,such as by a dedicated electric motor. The pump forces hydraulic fluidinto an inlet (not shown) of the housing 42. The inlet directs the flowof the fluid to a directional control valve 66.

The directional control valve 66 directs the fluid to an appropriatechamber portion 60 or 62 of the hydraulic motor 52. The flow ofhydraulic fluid toward one of the chamber portions 60 or 62 increasesthe pressure within that chamber portion 60 or 62. When the pressure ofone chamber portion 60 or 62 increases relative to the pressure of theother chamber portion 60 or 62, the piston 58 moves axially until thepressure within each chamber portion 60 or 62 again equalizes. As thepiston 58 moves axially, the volume of one chamber portion 60 or 62increases and the volume of the other chamber portion 60 or 62decreases. The decreasing chamber portion 60 or 62 is vented to allow aportion of the fluid contained in the decreasing chamber portion 60 or62 to escape. The escaping fluid exits the housing 42 via a return (notshown) and is directed into the reservoir.

An embodiment of the directional control valve 66 is shown in FIG. 4.The directional control valve 66 contains a valve core part 68 and avalve sleeve part 70. A portion of the valve core part 68 is containedwithin and is rotatable relative to the valve sleeve part 70.

The valve sleeve part 70 includes three radially directed passages 72that extend from an outer circumference of the valve sleeve part 70 toan inner circumference of the valve sleeve part. Each of these radialpassages 72 is supplied with hydraulic fluid that enters the housing 42through the inlet. Two axially extending grooves 74 and 76 areassociated with each radial passage 72. The axially extending grooves 74and 76 are located on the inner circumference of the valve sleeve part70. As shown in FIG. 4, one groove 76 is located clockwise from eachradial passage 72 and one groove 74 is located counter-clockwise fromeach radial passage. The grooves 74 and 76 are equidistant from arespective radial passage 72. Each groove 74 leads to a passage 78extending radially outwardly through the valve sleeve part 70. Eachgroove 76 leads to a passage 80 extending radially outwardly through thevalve sleeve part 70. Each groove 74 and 76 and associated passage 78and 80 is associated with a particular chamber portion 60 and 62 of thehydraulic motor 52. For example, with reference to FIG. 4, each groove76 and associated passage 80 located immediately clockwise of a radialpassage 72 will supply hydraulic fluid to chamber portion 62; whereas,each groove 74 and associated passage 78 located immediatelycounter-clockwise from a radial passage 72 will supply hydraulic fluidto chamber portion 60.

Six grooves 82 are located around the outer circumference of the valvecore part 68. The valve core part 68 also includes six protrusions 84 orlands. A protrusion 84 separates adjacent grooves 82 on the outercircumference of the valve core part 68. Side walls of the protrusion 84form side walls of the grooves 82.

When the valve core part 68 is located relative to the valve sleeve part70 such that each protrusion 84 of the valve core part 68 is centeredrelative to a respective groove 74 or 76 of the valve sleeve part 70,the directional control valve 66 is in a neutral position. FIG. 4illustrates the directional control valve 66 in the neutral position. Inthe neutral position, the pressure within each chamber portion 60 and 62of the hydraulic motor 52 is the same so that the piston 58 isstationary. When the valve core part 68 is rotated relative to the valvesleeve part 70, access to one of the two associated grooves 74 or 76 ofthe valve sleeve part 70 is restricted by a protrusion 84 of the valvecore part 68, while access to the other of the two associated grooves 74or 76 is increased. This allows a greater amount of the hydraulic fluidto flow toward the open groove 74 or 76, resulting in an increase inpressure of the respective chamber portion 60 or 62 associated with thatgroove 74 or 76. As a result of the increased pressure within therespective chamber portion 60 or 62, the piston 58 of the hydraulicmotor 52 is moved. For example, if the valve core part 68 is rotatedclockwise as viewed in FIG. 4, the groove 74 of the valve sleeve part 70located on the counter-clockwise side of the radial passage 72 becomesblocked and the groove 76 located on the clockwise side of the radialpassage 72 becomes open. Thus, a greater amount of the hydraulic fluidis directed toward the open groove 76. Pressure in the chamber portion62 of the hydraulic motor 52 associated with the open groove 76 isincreased relative to the pressure in chamber portion 60. As a result,the piston 58 is moved in an axial direction and rotates the sector gear46, causing the steerable wheels of the vehicle to be turned in theappropriate direction.

The piston 58 of the hydraulic motor 52 contains a bore 86 that is opentoward the directional control valve 66. The valve sleeve part 70 and afollow-up member 88 form an integral one-piece unit that is supportedfor rotation relative to the piston 58 by a plurality of balls 90. Theouter periphery of the follow-up member 88 is threaded. The plurality ofballs 90 interconnects the threaded outer periphery of the follow-upmember 88 with an internal thread 92 formed in the bore 86 of the piston58. As a result of the interconnecting plurality of balls 90, axialmovement of the piston 58 causes the follow-up member 88 and the valvesleeve part 70 to rotate. The rotation of the follow-up member 88 andthe valve sleeve part 70 returns the directional control valve 66 to theneutral position.

The valve core part 68 of the directional control valve 66 is fixedlyconnected to an input shaft 40 (FIG. 3). A first end 96 of a torsion bar94 is fixed relative to the input shaft 40 and the valve core part 68. Asecond end 98 of the torsion bar 94 is fixed relative to the valvesleeve part 70 and the follow-up member 88. At least a portion of thetorsion bar 94 extends through an axially extending bore 100 in thevalve core part 68, as shown in FIGS. 3-5.

When the resistance to turning of the steerable wheels of the vehicle isbelow a predetermined level, rotation of the input shaft 40 of the powersteering gear 34 is transferred through the torsion bar 94 and causesrotation of the follow-up member 88. As a result, the directionalcontrol valve 66 remains in the neutral position. Rotation of thefollow-up member 88 causes movement of the piston 58 and results inturning of the steerable wheels.

When resistance to turning the steerable wheels of the vehicle is at orabove the predetermined level, rotation of the follow-up member 88 isresisted. As a result, rotation of the input shaft 40 of the powersteering gear 34 rotates the first end 96 of the torsion bar 94 relativeto the second end 98 of the torsion bar. The rotation of the first end96 of the torsion bar 94 relative to the second end 98 of the torsionbar applies torsion across the torsion bar 94 and causes the valve corepart 68 to rotate relative to the valve sleeve part 70.

As discussed above, when the valve core part 68 rotates relative to thevalve sleeve part 70, hydraulic fluid is directed toward one of thechamber portions 60 and 62. As a result, the piston 58 moves within thechamber 56. Movement of the piston 58 results in turning of thesteerable wheels of the vehicle, as well as, rotation of the follow-upmember 88. As discussed above, rotation of the follow-up member 88rotates the valve sleeve part 70 until the directional control valve 66is again in the neutral position. When the directional control valve 66is in the neutral position, the torsion across the torsion bar 94 isremoved and the first end 96 of the torsion bar 94 is no longer rotatedrelative to the second end 98 of the torsion bar.

As shown in FIG. 5, the valve sleeve part 70 also includes first andsecond lugs 102 that are disposed in diametrically opposed cut-outs 104in the valve core part 68. Upon rotation of the valve core part 68 ofbetween 2° and 8° relative to the valve sleeve part 70, the lugs 102 ofthe valve sleeve part 70 engage the cut-outs 104 in the valve core part68 to cause the valve sleeve part 70 to be rotated along with the valvecore part 68. Such rotation of the valve sleeve part 70 causes thepiston 58 to move within the chamber 56 and, hence, allows for thesteerable wheels of the vehicle to be turned by the turning of the inputshaft 40 of the power steering gear 34. Thus, even if a loss inhydraulic fluid pressure has occurred, turning the input shaft 40 of thepower steering gear 34 enables the turning of the steerable wheels ofthe vehicle.

As shown schematically in FIGS. 1 and 2, the second assembly 26 alsoincludes at least one position sensor 32 for sensing rotation of theoutput shaft 50 of the drive mechanism 44 of the power steering gear 34.The position sensor 32 is preferably a non-contacting position sensor.Upon sensing the rotation of the output shaft 50, the position sensor 32generates a signal indicative of the rotation of the output shaft 50.

The second electronic control unit 28 receives the position signal fromthe position sensor 32. The second electronic control unit 28 isoperable to generate and transmit a second signal corresponding to theposition of the output shaft 50 of the drive mechanism 44 of the powersteering gear 34 that the position sensor 32 sensed.

In the illustrated embodiment, the first electronic control unit 22 iselectrically connected to the second electronic control unit 28 by acommunication wire 106. The communication wire 106 transfers the firstsignal generated by the first electronic control unit 22 to the secondelectronic control unit 28 and also transfers the second signalgenerated by the second electronic control unit 28 to the firstelectronic control unit 22. In one embodiment, the communication wire106 is a fiber optic cable and the first and second signals are opticalsignals. Alternatively, other forms of communication between the firstelectronic control unit 22 and the second electronic control unit 28 arecontemplated by the present invention. For example, wirelesscommunication or hard wiring between the first and second electroniccontrol units 22 and 28 may be used.

The first electronic control unit 22 receives the second signal. Inresponse to the second signal, the first electronic control unit 22controls the first electric motor 20 to control the steering resistanceapplied to the steering wheel 12. The first electronic control unit 22may run a known algorithm that uses the second signal and vehicle speedas parameters to determine the amount of resistance to apply to thesteering wheel 12. The first electric motor 20, through the gearassembly 24, applies a force to the shaft 14 to resist rotation of thesteering wheel 12.

The steering apparatus 10 also includes a mechanism 108. The mechanism108 includes a clutch 110 and a flexible cable 112, as is illustrated inFIGS. 1 and 2. The mechanism 108 includes two modes of operation. In thefirst mode of operation, illustrated in FIG. 1, the mechanism 108mechanically connects the steering wheel 12 to the input shaft 40 of thepower steering gear 34. In the second mode of operation, illustrated inFIG. 2, the mechanism 108 does not mechanically connect the steeringwheel 12 to the input shaft 40 of the power steering gear 34 and amechanical connection between the steering wheel 12 and the steeringgear 34 is lacking.

The clutch 110 of the mechanism 108 is a known device for engaging anddisengaging members. The clutch 110 illustrated schematically in FIGS. 1and 2 includes first and second members 114 and 116, respectively. Thefirst member 114 includes an upper surface 118 and a lower surface 120.The upper surface 118 of the first member 114 is fixed relative to theshaft 14 opposite the steering wheel 12. A lower surface 120 of thefirst member 114 includes a plurality of teeth 122.

The second member 116 also includes an upper surface 124 and a lowersurface 126. The upper surface 124 of the second member 116 includes aplurality of teeth 128 for meshingly engaging teeth 122 of the firstmember 114. The lower surface 126 of the second member 116 is fixed tothe flexible cable 112.

The second member 116 of the clutch 110 is supported relative to and isbiased toward the first member 114. A device (not shown) that forms apart of the clutch 110 is coupled to the second member 116 for movingthe second member 116 out of engagement with the first member 114. Thedevice may be an electric solenoid, a pneumatic cylinder, or any otherknown device for moving the second member 116 out of engagement with thefirst member 114. The clutch 110 is normally closed, meaning that whenthe device for moving the second member 116 out of engagement is notactuated, the first member 114 is in meshing engagement with the secondmember 116.

The clutch 110 is coupled to a power source (not shown). Preferably, thepower source is the vehicle battery or air supply. When the clutch 110receives electric, pneumatic, or other energy from the power source, thedevice for disengaging the second member 116 from the first member 114is actuated and, the second member 116 is moved out of meshingengagement with the first member 114.

When the clutch 110 is engaged, rotation of the steering wheel 12results in rotation of the second member 116. When the clutch 110 isdisengaged, the second member 116 is not rotated by rotation of thesteering wheel 12.

FIG. 6 schematically illustrates a portion of the clutch 110 with thefirst member 114 in engagement with the second member 116. When theclutch 110 is engaged, angled portions of the teeth 122 of the firstmember are received in angled portions of the second member 116 andangled portions of the teeth 128 of the second member are received inangled portions of the first member, as shown in FIG. 6 during rotationin the direction R. This engagement of the first and second members 114and 116 is sufficient to actuate the steering gear 34 of the secondassembly 26. This condition of the clutch 110 may occur, for example,when a portion of the first assembly 16 is not operating properly butthe second assembly 26 is operating properly.

However, if the second assembly 26 fails to operate properly, excessivetorque levels may be required for turning the steerable wheels. As aresult, the torque levels that must be transferred through the clutch110 increase. The increased torque levels result in the first and secondmembers 114 and 116 of the clutch 110 rotating relative to one anotherso that portions of the teeth 122 and 128 that extend perpendicular tothe direction of rotation R contact one another, as shown in FIG. 7. Therelative rotation results in a feel of looseness or play in the steeringwheel 12. The looseness gives the operator a physical indication thatmaintenance or repairs to the apparatus 10 may be necessary.

The flexible cable 112 of the mechanism 108 includes a first end portion130 and a second end portion 132. The first end portion 130 of theflexible cable 112 is fixed to the second member 116 of the clutch 110and is rotatable with rotation of the second member 116. The second endportion 132 of the flexible cable 112 is fixed to the input shaft 40 ofthe steering gear 34.

The flexible cable 112 is preferably a braided steel cable. Althoughradially flexible, the flexible cable 112 has a high torsional rigidity.The flexibility allows the flexible cable 112 to be easily routedbetween the first assembly 16 and the second assembly 26 by allowing theflexible cable 112 to be routed around and through vehicle componentsthat would interfere with a rigid connection. The flexible cable 112also allows the steering gear 34 to be mounted on a portion of thevehicle that is movable relative to the steering wheel 12. The hightorsional rigidity of the flexible cable 112 causes the second endportion 132 of the flexible cable 112 to rotate when the first endportion 130 of the flexible cable 112 is rotated.

During normal operation of the steering apparatus 10, power is suppliedto the clutch 110. As a result, the second member 116 of the clutch 110is disengaged from the first member 114 of the clutch and the mechanism108 is in the second mode of operation, as is illustrated in FIG. 2.When the mechanism 108 is in the second mode of operation, the steeringapparatus 10 is steered-by-wire and there is no mechanical connectionbetween the steering wheel 12 and the steering gear 34. It is noted thatwhen the clutch 110 is disengaged, rotation of the input shaft 40 of thepower steering gear 34 will rotate the second member 116 of the clutch110. However, since the clutch 110 is disengaged, rotation of the secondmember 116 will not cause rotation of the steering wheel 12.

In the event of a failure that causes the steer-by-wire operation of thesteering apparatus 10 to automatically shutdown or in the event of apurposeful disengagement of the steer-by-wire operation, the secondmember 116 of the clutch 110 moves into meshing engagement with firstmember 114 of the clutch. Thus, the mechanism 108 is in the first modeof operation and a mechanical connection is created between the steeringwheel 12 and the steering gear 34, as is illustrated in FIG. 1. When themechanism 108 is in the first mode of operation, rotation of thesteering wheel 12 is transferred through the clutch 110 and the flexiblecable 112 to the input shaft 40 of the steering gear 34 to enable manualactuation of the power steering gear 34. The mechanical connection alsoenables manual turning of the steerable wheels of the vehicle if thelugs 102 of the valve sleeve part 70 contact the valve core part 68.

Alternatively, the clutch 110 may be operatively coupled to the firstelectronic control unit 22. The first electronic control unit 22 maycontrol the mode of operation of the mechanism 108 in response to thesecond signal from the second electronic control unit 28. For example,if in response to the second signal, the first electronic control unit22 determines that the steering gear 34 is not being properly actuatedin response to the first signal, the first electronic control unit mayshutdown steer-by-wire operation and engage the first and second members114 and 116 of the clutch 110 to enable manual actuation of the steeringgear 34.

From the above description of the invention, those skilled in the artwill perceive improvements, changes and modifications. For example, whenoperating with the clutch 110 engaged as a result of a hydraulic failurein steering gear 34, the electric motors 20 and 30 may combine theirefforts to achieve a redundant power assist. Alternatively, either motor30 or motor 20 may operate to provide a power assist.

Also, electric motor 20 may be operated such that its resistance torqueincreases dramatically as the steering gear 34 nears its mechanical endof travel. This increased resistance will signal the operator to stopturning the steering wheel 12 before fluid is shut off to valve 66, thusmaintaining cooling fluid flow in the steering gear 34. Suchimprovements, changes and modifications within the skill of the art areintended to be covered by the appended claims.

Having described the invention, we claim the following:
 1. A steeringapparatus for turning steerable wheels of a vehicle in response torotation of a vehicle steering wheel, the apparatus comprising: a firstassembly operatively coupled to the steering wheel, the first assemblyincluding components for monitoring applied torque and angular rotationof the steering wheel and for transmitting a first signal indicative ofthe applied torque and angular rotation of the steering wheel; a secondassembly including a steering gear for, when actuated, turning thesteerable wheels of the vehicle and components for receiving the firstsignal and actuating the steering gear in response to the first signal;and a mechanism having first and second modes of operation, themechanism, when in the first mode of operation, providing a mechanicalconnection between the steering wheel and the steering gear for enablingmanual actuation of the steering gear, when the mechanism is in thesecond mode of operation, the steering wheel and the steering gearlacking a mechanical connection for enabling manual actuation of thesteering gear, the mechanism including a clutch have a first member anda second member, the second member being in meshing engagement with thefirst member when the mechanism is in the first mode of operation, thesecond member being disengaged from the first member when the mechanismis in the second mode of operation, the mechanism further including aflexible cable, the flexible cable being connected to one of the firstand second members of the clutch.
 2. The steering apparatus as definedin claim 1 wherein the flexible cable has a first end portion and asecond end portion, rotation of the first end portion of the flexiblecable resulting in rotation of the second end portion of the flexiblecable.
 3. The steering apparatus as defined in claim 1 wherein theflexible cable is a braided wire cable.
 4. The steering apparatus asdefined in claim 1 wherein the steering gear is hydraulically poweredand the flexible cable attaches to an input shaft of the steering gear,rotation of the input shaft directing flow of fluid in the steeringgear.
 5. A steering apparatus for turning steerable wheels of a vehiclein response to rotation of a vehicle steering wheel, the apparatuscomprising: a first assembly operatively coupled to the steering wheel,the first assembly including components for monitoring applied torqueand angular rotation of the steering wheel and for transmitting a firstsignal indicative of the applied torque and angular rotation of thesteering wheel; a second assembly including a steering gear for, whenactuated, turning the steerable wheels of the vehicle and components forreceiving the first signal and actuating the steering gear in responseto the first signal; and a mechanism having first and second modes ofoperation, the mechanism, when in the first mode of operation, providinga mechanical connection between the steering wheel and the steering gearfor enabling manual actuation of the steering gear, when the mechanismis in the second mode of operation, the steering wheel and the steeringgear lacking a mechanical connection for enabling manual actuation ofthe steering gear, the mechanism including a clutch have a first memberand a second member, the second member being in meshing engagement withthe first member when the mechanism is in the first mode of operation,the second member being disengaged from the first member when themechanism is in the second mode of operation, the meshing engagementbetween the first and second members of the clutch enabling a predefinedamount of relative rotation between the first and second members so asto provide a physical indication of improper operation of portions ofthe apparatus.
 6. The steering apparatus as defined in claim 5 whereinthe first member includes a first plurality of teeth, each tooth of thefirst plurality of teeth having an angled end surface, the second memberincluding a second plurality of teeth, adjacent teeth of the secondplurality of teeth being separated by an angled recess, the angled endsurface of each tooth of the first member being received in anassociated angled recess in the second member when the first and secondmembers are in meshing engagement.
 7. The steering apparatus as definedin claim 6 wherein the first plurality of teeth of the first member areseparated from the second plurality of teeth of the second member whenthe angled end surface of each tooth of the first member is received inthe associated angled recess in the second member.
 8. The steeringapparatus as defined in claim 7 wherein the predefined amount ofrelative rotation between the first and second members results inengagement of a side surface of each tooth of the first plurality ofteeth with a side surface of an associated tooth of the second pluralityof teeth.